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111 lines
4.2 KiB
HTML
111 lines
4.2 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>Function Multiversioning - Using the GNU Compiler Collection (GCC)</title>
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<meta http-equiv="Content-Type" content="text/html">
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<meta name="description" content="Using the GNU Compiler Collection (GCC)">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="C_002b_002b-Extensions.html#C_002b_002b-Extensions" title="C++ Extensions">
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<link rel="prev" href="C_002b_002b-Attributes.html#C_002b_002b-Attributes" title="C++ Attributes">
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<link rel="next" href="Namespace-Association.html#Namespace-Association" title="Namespace Association">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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Copyright (C) 1988-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being ``Funding Free Software'', the Front-Cover
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(see below). A copy of the license is included in the section entitled
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``GNU Free Documentation License''.
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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funds for GNU development.-->
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</head>
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<body>
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<div class="node">
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<a name="Function-Multiversioning"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Namespace-Association.html#Namespace-Association">Namespace Association</a>,
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Previous: <a rel="previous" accesskey="p" href="C_002b_002b-Attributes.html#C_002b_002b-Attributes">C++ Attributes</a>,
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Up: <a rel="up" accesskey="u" href="C_002b_002b-Extensions.html#C_002b_002b-Extensions">C++ Extensions</a>
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<hr>
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</div>
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<h3 class="section">7.8 Function Multiversioning</h3>
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<p><a name="index-function-versions-4235"></a>
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With the GNU C++ front end, for x86 targets, you may specify multiple
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versions of a function, where each function is specialized for a
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specific target feature. At runtime, the appropriate version of the
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function is automatically executed depending on the characteristics of
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the execution platform. Here is an example.
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<pre class="smallexample"> __attribute__ ((target ("default")))
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int foo ()
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{
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// The default version of foo.
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return 0;
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}
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__attribute__ ((target ("sse4.2")))
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int foo ()
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{
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// foo version for SSE4.2
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return 1;
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}
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__attribute__ ((target ("arch=atom")))
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int foo ()
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{
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// foo version for the Intel ATOM processor
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return 2;
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}
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__attribute__ ((target ("arch=amdfam10")))
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int foo ()
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{
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// foo version for the AMD Family 0x10 processors.
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return 3;
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}
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int main ()
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{
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int (*p)() = &foo;
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assert ((*p) () == foo ());
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return 0;
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}
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</pre>
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<p>In the above example, four versions of function foo are created. The
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first version of foo with the target attribute "default" is the default
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version. This version gets executed when no other target specific
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version qualifies for execution on a particular platform. A new version
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of foo is created by using the same function signature but with a
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different target string. Function foo is called or a pointer to it is
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taken just like a regular function. GCC takes care of doing the
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dispatching to call the right version at runtime. Refer to the
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<a href="http://gcc.gnu.org/wiki/FunctionMultiVersioning">GCC wiki on Function Multiversioning</a> for more details.
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</body></html>
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